The best way to do this is to set up a private 2-player match on any map with a rocket launcher. Damage yourselves until you're low on health and stand in front of each other (With a certain distance). Now person A needs to shoot and person B will do the same a few seconds after, so that person B dies and his rocket will kill person A while he is dead and not yet respawned. Repeat vice versa.

dont make it to difficult, the easiest way is just to pick up the "kamikaze" skull und press RB when enemy is near you --> achievement unlocked